Diamond sintered compact having high electrical conductivity and production method thereof

ABSTRACT

The present invention is to provide a diamond sintered compact having good conductivity together with the characteristics, such as hardness, thermal conductivity, thermal resistance, chemical stability, almost equal to those of a natural diamond. A boron-doped diamond sintered compact having good conductivity and high thermal resistance is produced by a sintering process, in which 90 to 99.9 wt. % of a boron-doped diamond powder and 0.1 to 10% wt. % of a powder comprising, one or more of carbonates including Mg, Ca, Sr or Ba, and/or one or more of composite carbonates composed by two or more of these elements, as a bonding phase component, are sintered together under Ht/HP conditions, and the bonding phase component melts and then fills into the space between the boron-doped diamond powder particles.

CROSS REFERENCED TO RELATED APPLICATION

This is a U.S. national phase application under U.S.C. §371 ofInternational Patent Application No. PCT/2007/070888, filed Oct. 26,2007 and claims the benefit of Japanese Application No. 2006-296902,filed Oct. 31, 2006; Japanese Application No. 2006-296903, filed Oct.31, 2006; Japanese Application No. 2007-261682, filed Oct. 5, 2007 andJapanese Application No. 2007-261683, filed Oct. 5, 2007. TheInternational Application was published in Japanese on May 8, 2008 asInternational Publication No. WO/2008/053796 under PCT Article 21(2).The contents of the above applications are incorporated herein in theirentirety.

TECHNICAL FIELD

The present invention relates to a production method to produce diamondsintered compacts having high electrical conductivity, in an effectiveand simplified manner; the invention also relates to a production methodfor producing boron-doped diamond sintered compacts having an excellentthermal resistance.

BACKGROUND OF THE INVENTION

Since, diamonds have inherent characteristics as high hardness, highthermal conductivity, high thermal resistance, and excellent chemicalstability;

diamonds have been utilized for many applications, such as, abrasionresistance materials, electronic device/sensor materials,biotechnological materials, and optical materials. Widely appliedmethods to produce diamonds are; the vapor-phase synthesis method usingvarious kinds of CVD processes, and/or a synthesis method using anultrahigh pressure/high temperature (HP/HT) apparatus.

It is well known that diamonds are inherently non-conductive material.However, in recent years, the boron-doped diamond has attracted noticedue to its semiconductor characteristics. The following are knownexamples of producing the boron-doped diamond; the vapor-phase synthesismethod in which a small amount of boron components are added to areaction gas while synthesizing the diamond; and the HP/HT synthesismethod in which under the conditions of a pressure in the range of 5 to10 GPa and a temperature in the range of 1300 to 2000° C., the diamondis synthesized from graphite powder and boron powder as the materialpowders.

Also, since diamonds inherently have characteristics such as hardnessand abrasion resistance, thus diamond sintered compacts are used invarious cutting tools. Such diamond sintered compacts are generallyproduced by sintering under HP/HT conditions. The following are knownexamples of producing the diamond sintered compact: a method in whichthe diamond-Co (cobalt) based sintered compact is produced from diamondpowder and Co powder as material powders; the material powders aresintered in the HP/HT apparatus under the conditions of a pressure ofabout 5.5 GPa and a temperature of about 1500° C. Another method inwhich the diamond-ceramics based sintered compact is produced fromdiamond powder, Ti (titanium) powder, Zr (zirconium) powder and Cr(chromium) powder as material powders; the material powders are sinteredin the HP/HT apparatus under the conditions of a pressure of 6.5 GPa orhigher and a temperature in the range of 1700 to 1900° C., then furtherheated up at a temperature of 2000° C. or higher: and a method in whichthe diamond-carbonate based sintered compact is produced from diamondpowder and carbonate powder as material powders; the material powdersare sintered in the HP/HT apparatus under the conditions of a pressurein the range of 6 to 12 GPa and a temperature in the range of 1700 to2500° C.

Known references are: Japanese Patent Publication No. 2004-193522;Japanese Patent Publication No. H04-312982; Japanese Translation of PCTPublication No. 2006-502955; Japanese Patent Publication No. H05-194031;and

Japanese Patent No. 2,795,738.

The inherent characteristics of the diamond sintered compact have beenutilized widely. For example, the aforementioned conventional art refersto a diamond-Co based sintered compact that has electrical conductivity,since the bonding phase in the diamond-Co based sintered compactconsists of Co metal. So this diamond-Co based sintered compact has anadvantage that the electrical discharge machining process is usable formachining this diamond-Co based sintered compact, but also has thedisadvantage of low thermal resistance. On the other hand, theaforementioned conventional art refers to a diamond-carbonate basedsintered compact that has the advantage of excellent thermal resistance;but due to its non-conductivity, the electrical discharge machiningprocess is not usable for machining this diamond-carbonate basedsintered compact. Therefore, there is a problem that only the laser beammachining process is usable for machining this diamond-carbonate basedsintered compact. Hence, in conventional arts, it is very difficult toobtain a diamond sintered compact having both good electricalconductivity and other inherent characteristics of a diamond, such ashardness, thermal conductivity, thermal resistance and chemicalstability. This is one of the limitations which prevent wide applicationof the diamond sintered compact.

The present invention relates to obtaining a diamond sintered compact,having both good conductivity and other characteristics of a naturaldiamond, such as, hardness, thermal conductivity, thermal resistance andchemical stability. The present invention also relates to a simplifiedand effective production method for the diamond sintered compact.

SUMMARY OF THE INVENTION

In order to solve the aforementioned problems, the inventers conductedintensive research on producing diamond sintered compacts, concerningboth the material powders and the sintering methods; and found thefollowing (a) to (d).

(a) Starting material powders for the sintered compact are: aboron-doped diamond powder made by doping a small amount of boroncomponent in a diamond; and as a component for forming a bonding phasein the sintered compact, a powder comprising, one or more of carbonatesincluding alkaline earth metal elements, Mg (magnesium), Ca (calcium),Sr (strontium) or Ba (barium), and/or one or more of compositecarbonates including two or more of these elements. Hereafter, thispowder is called an alkaline earth metal carbonate powder. A compositematerial powder, which has a layer structure of these starting materialpowders, or is a mixture of these starting material powders; is sinteredunder a HP/HT condition. The alkaline earth metal carbonate powder meltsat a temperature of about 2300° C.; and then a boron-doped diamondsintered compact, in which the space between the boron-doped diamondpowder particles is filled with the alkaline earth metal carbonatepowder, is produced by this process.

(b) Starting material powders for the sintered compact are: a diamondpowder; a boron powder; and as a component for forming a bonding phasein the sintered compact, a powder comprising, one or more of carbonatesincluding alkaline earth metal elements, Mg, Ca, Sr or Ba, and/or one ormore of composite carbonates including two or more of these elements.Hereafter, this powder is called an alkaline earth metal carbonatepowder. Predetermined quantities of these starting material powders aremixed to make a composite material powder. Sintering is preformed underHP/HT conditions in two steps; as the first step, a diffusion process isconduced, in which the boron in the mixed composite material powder isdiffused into the diamond powder under predetermined pressure andpredetermined temperature, as the second step, the alkaline earth metalcarbonate powder as the bonding phase component, melts under higherpressure and higher temperature than the previous step, and then fillsthe space between the boron-diffused diamond powder particles. Thus, adiamond sintered compact having good electric conductivity is producedby this process.

(c) A natural diamond inherently has very low electric conductivity of10⁻⁵ S/cm or less. On the other hand, boron-doped diamond powders haveremarkably high electric conductivity of about 1.5 S/cm. Although theaforementioned diamond-carbonate based sintered compact in theconventional art has low electric conductivity of about 10⁻⁵ S/cm; theboron-doped diamond sintered compacts produced by the method (a), andthe diamond sintered compact produced by the method (b), have anelectric conductivity in the range of about 1.0 to 10⁻² S/cm. Since thisvalue is roughly equivalent to the about 2×10⁻² S/cm which is theelectrical conductivity of the aforementioned diamond-Co based sinteredcompact; the products by (a) and (b) have good electrical conductivitysufficient for machining by the electrical discharge machining process.

(d) Sintered compacts, such as a diamond-Co based sintered compact,containing metal component as the bonding phase component, has limitedthermal resistance of about 700° C. On the other hand, the boron-dopeddiamond sintered compact produced by the method (a) and the diamondsintered compact produced by the method (b) have thermal resistance ofabout 1200° C., almost equivalent to the excellent thermal resistance ofthe carbonate based diamond sintered compact produced by using a naturaldiamond as raw material. Also, the products by (a) and (b) have goodhardness, good thermal conductivity and good chemical stability.

The present invention is based on the aforementioned (a) to (d), and hasthe features detailed in the following: A boron-doped diamond sinteredcompact wherein: a quantity in the range of from 90 to 99.9 weight % ofa boron-doped diamond powder in which the range of from 1 to 10 wt-% ofboron component is doped, and a quantity in the range of from 0.1 to 10wt-% of an alkaline earth metal carbonate powder including: one or moreof carbonates including alkaline earth metal elements Mg, Ca, Sr or Ba,and/or one or more of composite carbonates including two or more ofthese elements, as a bonding phase component in this sintered compact;these powders are sintered together under HP/HT conditions; and then thebonding phase component melts and fills the space between theboron-doped diamond powder particles.

A production method for a boron-doped diamond sintered compact wherein:the starting material powders are; a quantity in the range of from 90 to99.9 wt-% of a boron-doped diamond powder in which the range of from 1to 10 wt-% of boron component is doped, and a quantity in the range offrom 0.1 to 10 wt-% of an alkaline earth metal carbonate powder having,one or more of carbonates including alkaline earth metal elements Mg,Ca, Sr or Ba, and/or one or more of composite carbonates including twoor more of these elements, as a bonding phase component in this sinteredcompact; these starting material powders are sintered together in aHP/HT apparatus, under the conditions of a pressure in the range of from6.0 to 9.0 GPa and a temperature in the range of from 1600 to 2500° C.;and then the bonding phase component melts and fills the space betweenthe boron-doped diamond powder particles.

A production method for a diamond sintered compact having goodelectrical conductivity wherein: a composite material powder is amixture of a quantity of diamond powder in the range of from 80 to 99.4wt-%, a quantity of boron powder in the range of from 0.5 to 15 wt-%,and a quantity in the range of from 0.1 to 10 wt-% of an alkaline earthmetal carbonate powder including one or more of carbonates includingalkaline earth metal elements Mg, Ca, Sr or Ba, and/or one or more ofcomposite carbonates including two or more of these elements, as abonding phase component in the sintered compact; and the compositematerial powder is sintered in the HP/HT apparatus, as the first step ofthis production process, the boron component in the composite materialpowder is diffused into the diamond powder under the conditions of apressure in the range of from 5.0 to 8.0 GPa and a temperature in therange of from 1300 to 1800° C., after that as the second step, thebonding phase component melts under the conditions of a pressure in therange of from 6.0 to 9.0 GPa and a temperature in the range of from 1600to 2500° C., and then fills the space between the boron-doped diamondpowder particles.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

In an aspect of the present invention, the boron-doped diamond powderand its production method are described as follows: Boron-doped diamondpowder A boron-doped diamond powder produced by well known methods, forexample the vapor-phase synthesis method or the HP/HT synthesis method,is utilizable as a starting material powder. Although the quantity ofthe doped boron is not particularly restricted, but from a practicalviewpoint, the preferable range is from 1 to 10 wt-%. In other words,the solution of the equation, (Boron Weight)/[(Boron Weight)+(DiamondWeight)]×100, should be within a range of 1 to 10 wt-%, more preferablywithin a range of from 2 to 7 wt-%.

Alkaline earth carbonate powder. Starting material powders; aboron-doped diamond powder, and an alkaline earth metal carbonate powdercomprising, one or more of carbonates including alkaline earth metalelements, Mg, Ca, Sr or Ba, and/or one or more of composite carbonatesincluding two or more of these elements; are sintered together underHP/HT conditions. Under the condition of a temperature of about 2300°C., the alkaline earth metal carbonate powder melts, and then fills thespace between the diamond powder particles. Hence, the alkaline earthmetal carbonate powder aids to bond each adjacent diamond particlestogether, fills the space between the particles, and becomes the bondingphase for increasing the density of the sintered compact.

Combination ratio of the starting material powders. The ratio of theboron-doped diamond powder is in a range of from 90 to 99.9 wt-%. Thisrange aims to give the boron-doped diamond sintered compact of thepresent invention high electric conductivity together with the excellentinherent characteristics of a diamond, namely, hardness, thermalconductivity and chemical stability. On the other hand, the ratio of thealkaline earth metal carbonate powder is in a range of from 0.1 to 10wt-%. This range aims also to give the boron-doped diamond sinteredcompact of the present invention excellent thermal resistance togetherwith the predetermined high electric conductivity and the predetermineddensity of the sintered compact. If the ratio of the boron-doped diamondpowder is less than 90 wt-%, or the ratio of the alkaline earth metalcarbonate powder is more than 10 wt-%; the boron-doped diamond sinteredcompact will not have the predetermined high electric conductivity, andits characteristics as a sintered compact, namely, density, hardness,thermal conductivity and chemical stability, also will become poor. Onthe other hand, if the ratio of the alkaline earth metal carbonatepowder is less than 0.1 wt-%, or the ratio of the boron-doped diamondpowder is more than 99.9 wt-%; due to the decrease of the bonding phasecomponent, the characteristics of the sintered compact, namely,sinterability, strength and thermal resistance, will become poor.

Sintering conditions of the HP/HT apparatus. A sintered compact,produced in the HP/HT apparatus under a pressure lower than 6.0 GPa, hasinsufficient densification. A sintered compact, produced in the HP/HTapparatus at a pressure of 9.0 GPa, has practically sufficientdensification. The cost of a HP/HT apparatus having a function over 9.0GPa operating pressure is very expensive. Therefore, the pressure isdetermined in a range of from 6.0 to 9.0 GPa. If the heating temperatureof the HP/HT apparatus is lower than 1600° C., the carbonate does notcompletely melt and fill the space between the particles; since thesintering reaction is insufficient, the resulting sintered compact hasnot high density. On the other hand, if the heating temperature of theHP/HT apparatus is higher than 2500° C.; the temperature becomesexcessive for sintering the powders: under this condition, the diamondpowder is undesirably inverted to graphite. Therefore, the heatingtemperature range of from 1600 to 2500° C. is determined. The compositematerial powders for placing in the HP/HT apparatus should preferably bea layer structure of the boron-doped diamond powder and the alkalineearth metal carbonate powder. Or, a mixture of the boron-doped diamondpowder and the alkaline earth metal carbonate powder is usable as thecomposite material powder for placing in the HP/HT apparatus.

In another aspect of the present invention, the method for producing thediamond sintered compact is described as follows: Diamond powder andboron powder A diamond powder produced by the vapor-phase synthesismethod or other well known methods is utilizable as the diamond powderfor this method. A crystalline boron powder or an amorphous boron powderis suitable as the boron powder for this method. However, in some cases,a boron carbide powder with high boron content is also utilizable as theboron powder for this method. The HP/HT conditions, on the first step ofthis method, are of a pressure in the range of from 5.0 to 8.0 GPa and atemperature in the range of from 1300 to 1800° C. Under theseconditions, boron component in this boron powder is diffused over thesurface of the diamond powder. Therefore, the diamond powder, producedby this process, has the property which contributes to the electricalconductivity of the finished diamond sintered compact.

Alkaline earth carbonate powder. A mixture of the diamond powder, theboron powder and the alkaline earth carbonate powder becomes thecomposite material powder to be sintered under HP/HT conditions. Thesintering is preformed under HP/HT conditions in the below two steps. Asthe first step, the conditions are of a pressure in the range of from5.0 to 8.0 GPa and a temperature in the range of from 1300 to 1800° C.Under these conditions, the boron component is diffused into thediamond. As the second step, the conditions are of a pressure in therange of from 6.0 to 9.0 GPa and a temperature in the range of from 1600to 2500° C. Under these conditions, the bonding phase component melts,fills the space between the boron-doped diamond powder particles, andbecomes the bonding phase for increasing the density of the sinteredcompact.

Combination ratio of the starting material powders. The ratio of theboron-doped diamond powder is in a range of from 80 to 99.4 wt-%, andthe ratio of the boron powder is in a range of from 0.5 to 15 wt-%.These ranges aim to give the diamond sintered compact of the presentinvention high electric conductivity together with the excellentinherent characteristics of a diamond, namely, hardness, thermalconductivity and chemical stability. Concurrently, the ratio of thealkaline earth metal carbonate powder is in a range of from 0.1 to 10wt-%. This range aims to give the diamond sintered compact of thepresent invention high electric conductivity together with excellentthermal resistance and predetermined density of the sintered compact. Ifthe ratio of the diamond powder is less than 80 wt-%, or the ratio ofthe boron powder is less than 0.5 wt-%, or the ratio of the alkalineearth metal carbonate powder is more than 10 wt-%; the diamond sinteredcompact will not have the predetermined high electric conductivity; andits characteristics as a sintered compact, namely, hardness, thermalconductivity, chemical stability and density, also will become poor.

Alternatively, if the ratio of the alkaline earth metal carbonate powderis less than 0.1 wt-%, or the ratio of the boron-doped diamond powder ismore than 99.4 wt-%, or the ratio of the boron powder is more than 15 wt%; due to the decrease of the bonding phase component, thecharacteristics of the sintered compact, namely, sinterability, strengthand thermal resistance, will become poor. Additionally, a case; whereinthe diamond powder, the boron powder and the alkaline earth carbonatepowder are not directly mixed, but the boron-doped diamond powderbeforehand synthesized by the method shown in the cited prior arts andthe alkaline earth metal carbonate powder are mixed; can be supposed asa process for making a combination of the starting material powders.However, in this case, the above boron-doped diamond tends to containimpurities which are remains of metal catalysts used in its synthesizingprocess. This causes an adverse effect to the finished sinteredcompact's characteristics: for example, thermal resistance becomes poor.Furthermore, this synthesized boron-doped diamond forms lumps.Therefore, after synthesizing this boron-doped diamond, the lumps haveto be crushed, and then the remaining metal impurities have to beremoved by chemical treatments, then it divided into classes. Thissubsequent processing takes much time. Therefore, a combination of thediamond powder, the boron powder and the alkaline earth carbonate powderby each their pre-determined ratio, is have to be used as the compositematerial powder for the sintered compact. By using this combination asthe composite material powder for the sintered compact; the diamondparticles become free from the impurities, and also the subsequentprocessing is not needed. Additionally, the ratio of diamond and boronin the sintered compact can be controlled easily and accurately bychanging the combination ratio of the starting material powders.

Sintering conditions in the HP/HT apparatus. As the first step of thesintering process in the HP/HT apparatus, the conditions are of apressure in the range of from 5.0 to 8.0 GPa and a temperature in therange of from 1300 to 1800° C. Under these conditions, the boroncomponent is diffused into the diamond powder. This process contributesto the electrical conductivity of the diamond sintered compact. If thepressure and/or the heating temperature are lower than the above ranges,the boron component is not sufficiently diffused, thus leading to poorconductivity of the sintered compact.

On the other hand, if the pressure and/or the heating temperature arehigher than the above ranges; the carbonate component start to melt, andthen the diamond is undesirably sintered, although the boron diffusionphase has not been fully formed. Therefore, a pressure in the range offrom 5.0 to 8.0 GPa and a heating temperature in the range of from 1300to 1800° C. are determined as the conditions for the first step of thesintering process. As the second step of the sintering process in theHP/HT apparatus, a sintered compact, produced under a pressure lowerthan 6.0 GPa, has insufficient densification; and a sintered compact,produced at a pressure of 9.0 GPa, has sufficient densification. Thecost of a HP/HT apparatus having a function over 9.0 GPa operatingpressure is very expensive. From these factors, the pressure isdetermined in the range of from 6.0 to 9.0 GPa. If the heatingtemperature is lower than 1600° C., the alkaline earth metal carbonatedoes not completely melt and fill the space between the particles; thesintering reaction is also insufficient, the resulting sintered compacthas not high density.

On the other hand, if the heating temperature is higher than 2500° C.;the temperature becomes excessive for sintering the powders; under thiscondition, the diamond particles are undesirably changed to graphite.Therefore, the heating temperature has been determined in the range offrom 1600 to 2500° C. In the present invention, the composite materialpowder for placing in the HP/HT apparatus should preferably be a simplemixture of the diamond powder, the boron powder and the alkaline earthmetal carbonate powder. However, a composite material powder; which isnot only a simple mixture of the starting material powders, but has alayer structure of each starting material powder, the diamond powder,the boron powder and the alkaline earth metal carbonate powder; is alsousable for placing in the HP/HT apparatus.

In the present invention relating to the boron-doped diamond sinteredcompact and it's production method: the boron-doped diamond powder andthe alkaline earth metal carbonate powder for forming the bonding phasein the sintered compact are heated together under a HP/HT condition; thebonding phase component melts, and then fills the space between theboron-doped diamond powder particles; and the boron-doped diamondsintered compact thus produced, has not only good hardness, good thermalconductivity and good chemical stability, has but also good electricalconductivity and good thermal resistance.

Furthermore, the diamond powder, the boron powder and the alkaline earthmetal carbonate powder for forming the bonding phase in the sinteredcompact; are heated under HP/HT condition. As the first step, boron isdiffused into the diamond to give the sintered compact an electricalconductivity. As the second step, the bonding phase component melts, andthen fills the space between the boron-doped diamond powder particles.Thus, the diamond sintered compact with good electric conductivity isobtained by a simple and effective two step process.

The actual examples of the boron-doped diamond powders and the alkalineearth metal carbonate powders used in the invention are shown in Table1.

EXAMPLE 1

Table 1 shows the starting material powders, namely, various boron-dopeddiamond powders and various alkaline earth metal carbonate powders.Table 2 shows the composite material powders 1 to 13 prepared by makingthe combinations of the starting material powders shown in Table 1, andtheir combination ratios. The composite material powders 1 to 13 wereplaced into a general belt type HP/HT apparatus, in a layer structurewhere the alkaline earth metal carbonate powder[s] was[were] located ata lower layer and the boron-doped diamond powder was located at an upperlayer. Next, these composite material powders 1 to 13 were sinteredunder the sintering conditions (A) to (D) shown in Table 3. Then, theboron-doped diamond sintered compacts of this invention were produced asthe invention sintered compacts 1 to 15 shown in Table 4. The measuredcharacteristics of these invention sintered compacts 1 to 15, namely,electrical conductivity (electrical resistance), thermal resistance,hardness, thermal conductivity and chemical stability, are also shown inTable 4.

COMPARATIVE EXAMPLE 1

The composite material powders 21 to 24 for comparison shown in Table 5were prepared. The composite material powders 21 and 23 were in a layerstructure where the bonding phase component powders were located at alower layer and the diamond powder was located at an upper layer. Thecomposite material powders 22 and 24 were simple mixtures of the diamondpowder and the bonding phase component powder. These composite materialpowders 21 to 24 were placed into a general belt type HP/HT apparatus,and then sintered under the sintering conditions (E) and (F) shown inTable 6. Thus, the diamond sintered compacts for comparison wereproduced as the comparative sintered compacts 21 to 24 shown in Table 7.The measured characteristics of the comparative sintered compacts 21 to24 are also shown in Table 7.

In Example 1 and Comparative Example 1; the evaluation tests for thecharacteristics, namely, electrical conductivity (electricalresistance), thermal resistance and chemical stability; have beenperformed as follows.

Electrical conductivity evaluation test:

Four-point method was applied for measuring electrical conductivity ofthe samples.

Thermal resistance evaluation test:

The samples were heated in a vacuum furnace at the temperature of 800°C. for 30 minutes and then at the temperature of 1200° C. for 30minutes. After these heating processes, XRD (X-ray) analysis wasconduced for checking whether there was graphite (which is an inverseform of diamond) in the samples.

Chemical stability evaluation test:

The samples were soaked in heated fluoric acid at the temperature of150° C. for 2 hours. After this chemical treatment, a visual inspectionwas conduced for checking whether any shape deformation occurred in thesample sintered compacts.

TABLE 1 Mean diameter of Kind of powder Powder symbol particle [μm]Boron-doped powder BD (1) 0.5 BD (2) 3 BD (3) 8 Alkaline earth MgCO₃ 30carbonate powder CaCO₃ 50 SrCO₃ 30 BaCO₃ 30 (Mg, Ca)CO₃ 50 (Mg, Sr)CO₃50 Note: The doped quantities of boron in BD (1) to BD (3) arerespectively 2 wt %, 4 wt % and 7 wt %.

TABLE 2 Composite Boron-doped material diamond powder Alkaline earthcarbonate powder powder Powder Combination Combination Combinationsymbol symbol ratio [wt %] Powder symbol ratio [wt %] Powder symbolratio [wt %] 1 BD (1) 90 MgCO₃ 5 BaCO₃ 5 2 BD (1) 97 MgCO₃ 3 — — 3 BD(1) 90 CaCO₃ 2 MgCO₃ 8 4 BD (2) 98 MgCO₃ 2 — — 5 BD (2) 95 (Mg, St)CO₃ 5— — 6 BD (2) 90 SrCO₃ 5 BaCO₃ 5 7 BD (3) 95 CaCO₃ 5 — — 8 BD (3) 95MgCO₃ 2 BaCO₃ 3 9 BD (3) 95 (Mg, Ca)CO₃ 5 — — 10 BD (3) 95 (Mg, St)CO₃ 3(Mg, Ca)CO₃ 2 11 BD (2) 99.9 CaCO₃ 0.1 — — 12 BD (3) 99 MgCO₃ 0.5 CaCO₃  0.5 13 BD (1) 99.5 SrCO₃ 0.5 — —

TABLE 3 Sintering Temperature condition Pressure rise rate TemperatureHeat holding symbol [GPa] [° C./min.] [° C.] time [min.] (A) 8 200 230030 (B) 8 200 2000 30 (C) 7 200 1700 30 (D) 6 200 1600 30 Note:Temperature [° C.] was hold while Heat holding time [min.].

TABLE 4 Composite Sintered material Sintering Electrical Thermal compactpowder condition resistance Thermal Hardness conductivity Chemicalsymbol symbol symbol [Ω · cm] resistance [GPa] [cal/cm · sec · ° C.]stability Invention 1 (A) 10 No 65 1.7 ◯ sintered graphite compact 1Invention 2 (A) 0.8 No 68 1.9 ◯ sintered graphite compact 2 Invention 3(A) 13 No 66 1.7 ◯ sintered graphite compact 3 Invention 4 (B) 3 No 702.0 ◯ sintered graphite compact 4 Invention 5 (B) 12 No 68 1.7 ◯sintered graphite compact 5 Invention 6 (B) 15 No 66 1.7 ◯ sinteredgraphite compact 6 Invention 7 (C) 65 No 63 1.8 ◯ sintered graphitecompact 7 Invention 8 (C) 70 No 65 1.5 ◯ sintered graphite compact 8Invention 9 (C) 34 No 62 1.8 ◯ sintered graphite compact 9 Invention 10(C) 48 No 66 1.8 ◯ sintered graphite compact 10 Invention 11 (A) 1 No 701.8 ◯ sintered graphite compact 11 Invention 12 (B) 23 No 68 1.8 ◯sintered graphite compact 12 Invention 13 (C) 56 No 68 1.8 ◯ sinteredgraphite compact 13 Invention 3 (D) 60 No 45 1.7 ◯ sintered graphitecompact 14 Invention 11 (D) 14 No 49 1.8 ◯ sintered graphite compact 15Note: Symbol ◯ in Chemical stability section means that these sinteredcompacts were with no occurrence of shape deformation through thechemical stability evaluation test.

TABLE 5 Composite Diamond powder Bonding phase component material Meandiameter Mean diameter Mean diameter powder of particle Combination ofparticle Combination of particle Combination symbol [μm] ratio [wt %]Component [μm] ratio [wt %] Component [μm] ratio [wt %] 21 8 95 Co 1.5 5— — — 22 30 90 Co 1.5 10 — — — 23 8 95 MgCO₃ 20 3 BaCO₃ 40 2 24 30 90MgCO₃ 40 5 BaCO₃ 20 5

TABLE 6 Sintering Temperature condition Pressure rise rate TemperatureHeat holding symbol [GPa] [° C./min.] [° C.] time [min.] (E) 6 200 160030 (F) 8 200 2200 30 Note: Temperature [° C.] was hold while Heatholding time [min.].

TABLE 7 Composite Sintered material Sintering Electrical Thermal compactpowder condition resistance Thermal Hardness conductivity Chemicalsymbol symbol symbol [Ω · cm] resistance [GPa] [cal/cm · sec · ° C.]stability Comparative 21 (E) 0.8 Graphite 54 1.8 X sintered compact 21Comparative 22 (E) 30   Graphite 56 1.7 X sintered compact 22Comparative 23 (F) Immeasurable No 68 1.8 ◯ sintered graphite compact 23Comparative 24 (F) Immeasurable No 70 1.9 ◯ sintered graphite compact 24Note 1: Symbol ◯ in Chemical stability section means that these sinteredcompacts were with no occurrence of shape deformation through thechemical stability evaluation test. Symbol X means that these sinteredcompacts became powdered by flowing of Co phase. Note 2: Immeasurablemeans that their electrical resistances were very high, almost same thatof a natural diamond. I.e., their electrical conductivities were lessthan 10⁻⁵ [S/cm].

The measured characteristics of the present invention sintered compacts1 to 15 on Table 4 show obvious facts that these present inventionsintered compacts 1 to 15 have; good electric conductivity lower than 70Ω·cm, good thermal resistance with no occurrence of graphite through thethermal resistance evaluation test, good chemical stability with nooccurrence of shape deformation through the chemical stabilityevaluation test, and furthermore good hardness and good thermalresistance almost equal to those of a diamond.

On the other hand, the measured characteristics of the comparativesintered compacts 21 to 24 on Table 7 show obvious facts that thecomparative sintered compacts 21 and 22 have good electricalconductivity, but have poor thermal resistance due to occurrence ofgraphite through the thermal resistance evaluation test. Also, thechemical stabilities of the 21 and 22 are poor while the chemicalstability evaluation test, Co phase as the bonding phase was flowing outand then the sintered compacts became powdered. Additionally, thecomparative sintered compacts 23 and 24 have very high electricalresistances too high to measure, which are almost same level of anatural diamond. In other words, the 23 and 24 have no electricalconductivity. Therefore, the diamond sintered compact having both goodelectric conductivity and good thermal resistance, is obtained by thepresent invention. Additionally the electrical discharge machiningprocess is possible to use for machining this diamond sintered compact.As this diamond sintered compact will be finding wide application invarious industrial fields, the practical effects brought by the presentinvention are very significant.

EXAMPLE 2

Table 8 shows the starting material powders, namely, various diamondpowders, various boron powders, and various alkaline earth metalcarbonate powders. Table 9 shows the composite material powders 31 to 45prepared by making the combinations of the starting material powdersshown in Table 8, and their combination ratios. The composite materialpowders 31 to 45 were placed into a general belt type HP/HT apparatus.Next, to produce the products of the invention methods 31 to 45, thesecomposite material powders 31 to 45 were sintered under the sinteringconditions (G) to (K) including each first sintering step and eachsecond sintering step shown in Table 10. Then, the diamond sinteredcompacts of this invention were produced as the invention sinteredcompacts 31 to 45 shown in Table 11. The measured characteristics ofthese invention sintered compacts 31 to 45 which are respectivelyproducts of the invention methods 31 to 45, such as electricalconductivity (electrical resistance), thermal resistance, hardness,thermal conductivity and chemical stability, are also shown in Table 11.

EXAMPLE 2

The aforementioned composite material powders 31, 35 and 40 weresintered under the sintering conditions shown in Table 12 as thecomparative methods 31, 35 and 40. Thus, the diamond sintered compactsfor comparison were produced as the comparative sintered compacts 31, 35and 40 shown in Table 13. The measured characteristics of thesecomparative sintered compacts are also shown in Table 13. Furthermore,as references, the measured characteristics of the prior sinteredcompact 1 and 2 are also shown in Table 13. The prior sintered compact 1is a boron-doped diamond sintered compact produced by the prior method 1in Japanese Patent Publication No. 2006-502955. The prior sinteredcompact 2 is a diamond-carbonate based sintered compact produced by theprior method 2 in Japanese Patent No. 2,795,738.

In Example 2 and Comparative Example 2; the evaluation tests for thecharacteristics, namely, electrical conductivity (electricalresistance), thermal resistance and chemical stability; have beenperformed as follows.

Electrical conductivity evaluation test:

Four-point method was applied for measuring electrical conductivity ofthe samples.

Thermal resistance evaluation test:

The samples were heated in a vacuum furnace at the temperature of 800°C. for 30 minutes and then at the temperature of 1200° C. for 30minutes. After these heating processes, XRD (X-ray) analysis wasconduced for checking whether there was graphite (which is an inverseform of diamond) in the samples.

Chemical stability evaluation test:

The samples were soaked in heated fluoric acid at the temperature of150° C. for 2 hours. After this chemical treatment, a visual inspectionwas conduced for checking whether any shape deformation occurred in thesample sintered compacts.

TABLE 8 Mean diameter of Kind of powder Powder symbol particle [μm]Diamond powder D (1) 1.0 D (2) 3.0 D (3) 8.0 Boron-doped powder B (1)0.5 B (2) 3.0 B (3) 8.0 Alkaline earth MgCO₃ 30 carbonate powder CaCO₃50 SrCO₃ 30 BaCO₃ 30 (Mg, Ca)CO₃ 50 (Mg, Sr)CO₃ 50

TABLE 9 Composite material Diamond powder Boron powder Alkaline earthmetal carbonate powder powder Powder Combination Powder CombinationCombination Powder Combination symbol symbol ratio [wt %] symbol ratio[wt %] Powder symbol ratio [wt %] symbol ratio [wt %] 31 D (1) 90 B (1)5 MgCO₃ 3 BaCO₃ 2 32 D (2) 93 B (1) 4 MgCO₃ 3 — — 33 D (3) 90 B (1) 4CaCO₃ 2 MgCO₃ 4 34 D (4) 97 B (2) 2 CaCO₃ 1 — — 35 D (5) 96 B (2) 2SrCO₃ 1 MgCO₃ 1 36 D (6) 94 B (2) 3 (Mg, Ca)CO₃ 3 — — 37 D (7) 95 B (3)2 MgCO₃ 2 CaCO₃ 1 38 D (8) 90 B (3) 4 (Mg, Sr)CO₃ 6 — — 39 D (9) 92 B(3) 6 BaCO₃ 2 — — 40 D (10) 98 B (3) 1 CaCO₃ 1 — — 41 D (11) 99.4 B (1)0.5 MgCO₃ 0.1 — — 42 D (12) 88 B (1) 2 (Mg, Ca)CO₃ 10 — — 43 D (13) 85 B(2) 12 CaCO₃ 2 MgCO₃ 1 44 D (14) 83 B (2) 8 SrCO₃ 9 — — 45 D (15) 80 B(3) 15 BaCO₃ 3 CaCO₃ 2

TABLE 10 Sintering Temperature First sintering step Temperature Secondsintering step condition rise rate Pressure Temperature Heat holdingrise rate Pressure Temperature Heat holding symbol [° C./min.] [GPa] [°C.] time [min.] [° C./min.] [GPa] [° C.] time [min.] (G) 200 5.5 1600 30200 7 2000 15 (H) 200 6.5 1600 30 200 8 2200 20 (I) 200 7 1700 30 200 82000 20 (J) 200 5.0 1300 30 200 6 2300 20 (K) 200 8.0 1800 30 200 8 230020 Note: Temperature [° C.] was hold while Heat holding time [min.].

TABLE 11 Composite Sintered material Sintering Electrical Thermalcompact powder condition resistance Thermal Hardness conductivityChemical symbol symbol symbol [Ω · cm] resistance [GPa] [cal/cm · sec ·° C.] stability Invention 31 (G) 14.7 No 64 1.7 ◯ sintered graphitecompact 31 Invention 32 (G) 2.3 No 65 1.9 ◯ sintered graphite compact 32Invention 33 (G) 21 No 64 1.7 ◯ sintered graphite compact 33 Invention34 (I) 0.8 No 70 1.8 ◯ sintered graphite compact 34 Invention 35 (I) 8.4No 69 1.8 ◯ sintered graphite compact 35 Invention 36 (I) 3.5 No 66 1.7◯ sintered graphite compact 36 Invention 37 (H) 18.1 No 68 1.8 ◯sintered graphite compact 37 Invention 38 (H) 12.9 No 62 1.5 ◯ sinteredgraphite compact 38 Invention 39 (H) 0.5 No 64 1.7 ◯ sintered graphitecompact 39 Invention 40 (H) 30 No 70 1.8 ◯ sintered graphite compact 40Invention 41 (J) 1.0 No 62 1.8 ◯ sintered graphite compact 41 Invention42 (J) 1.2 No 60 1.8 ◯ sintered graphite compact 42 Invention 43 (J) 5.4No 63 1.7 ◯ sintered graphite compact 43 Invention 44 (K) 4.8 No 65 1.6◯ sintered graphite compact 44 Invention 45 (K) 12 No 66 1.9 ◯ sinteredgraphite compact 45 Note: Symbol ◯ in Chemical stability section meansthat these sintered compacts were with no occurrence of shapedeformation through the chemical stability evaluation test.

TABLE 12 Composite material Temperature First sintering stepsTemperature Second sintering steps Comparative powder rise rate PressureTemperature Heat holding rise rate Pressure Temperature Heat holdingmethods symbol [° C./min.] [GPa] [° C.] time [min.] [° C./min.] [GPa] [°C.] time [min.] Comparative 31 — — — — 200 6.5 1800 20 method 31Comparative 35 200 5.5 1800 20 — — — — method 35 Comparative 40 200 3.72000 30 200 5.5 2300 30 method 40 Note: Temperature [° C.] was holdwhile Heat holding time [min.].

TABLE 13 Sintered Electrical Thermal compact resistance Thermal Hardnessconductivity Chemical symbol [Ω · cm] resistance [GPa] [cal/cm · sec · °C.] stability Comparative 0.7 × 10⁴ Graphite 35 1.2 X sintered compact31 Comparative   6 × 10⁶ Graphite 28 0.7 X sintered compact 35Comparative 1.6 × 10⁶ Graphite 30 1.1 X sintered compact 40 Priorsintered 58 No graphite 55 1.8 Δ compact 1 Prior sintered ImmeasurableNo graphite 68 1.7 ◯ compact 2 Note 1: Symbol ◯ in Chemical stabilitysection means that these sintered compacts were with no occurrence ofshape deformation through the chemical stability evaluation test. SymbolX means that binding phases in these sintered compacts flowed out due totheir weak bonding forces. Symbol Δ means that binding phases in thesesintered compacts partly flowed out. Note 2: Immeasurable means thattheir electrical resistances were very high, almost same that of anatural diamond. I.e., their electrical conductivities were less than10−5 [S/cm].

On comparison between: Table 11 in which the characteristics of thepresent invention sintered compacts 31 to 45 are shown; and Table 13 inwhich the characteristics of the comparative sintered compacts 31, 35and 40, and the characteristics of the prior sintered compact 1 and 2,are shown; the following facts become obvious. The present inventionsintered compacts 31 to 45, which are respectively products of theinventive methods 31 to 45, have good electric conductivity, goodthermal resistance, and furthermore good hardness and good thermalresistance almost equal to those of a natural diamond.

On the other hand, concerning the comparative sintered compacts 31, 35and 40, and the prior sintered compact 1 and 2; at least one of theircharacteristics, namely, electrical conductivity, thermal conductivity,thermal resistance, hardness and chemical stability, is worse than thatof the present invention sintered compacts 31 to 45. The comparativesintered compacts 31, 35 and 40 are respectively products of thecomparative methods 31, 35 and 40, in which their production conditionsare outside those of the invention methods. The prior sintered compact 1and 2, are respectively products of the prior methods 1 and 2.Therefore, the diamond sintered compact having both good electricconductivity and good thermal resistance is obtained by the presentinvention in an effective and simplified manner. Additionally, thediamond sintered compact produced by the present invention conditionshave good electrical conductivity enough for the electrical dischargemachining process is possible to use for machining these diamondsintered compacts. As this diamond sintered compact will be finding wideapplication in various industrial fields, the practical effects broughtby the present invention are very significant.

The boron-doped diamond sintered compact and its production method inthe aspect 1 of the present invention teach the following. Theboron-doped diamond powder and the alkaline earth carbonate powdercontaining the component for forming the bonding phase in the sinteredcompact, are sintered together under HP/HT condition: under thiscondition, the bonding phase component melts and fills the space betweenthe boron-doped diamond powder particles; and then the boron-dopeddiamond sintered compact is produced. This boron-doped diamond sinteredcompact has good characteristics, namely, good hardness, good thermalconductivity and good chemical stability. Additionally, this boron-dopeddiamond sintered compact has good electrical conductivity and goodthermal resistance. Therefore, this boron-doped diamond sintered compacthas the characteristics almost equal to those of natural diamonds,together with an excellent machinability that the electrical dischargemachining process is easily possible to use for machining thisboron-doped diamond sintered compact. As this diamond sintered compactwill be finding wide application in various industrial fields, thepractical effects brought by the present invention are very significant.

The production method of the diamond sintered compact in another aspectof the present invention is the following. The diamond powder, the boronpowder and the alkaline earth carbonate powder containing the componentfor forming the bonding phase in the sintered compact, are sinteredtogether under HP/HT condition: as the first step of this process, theboron is diffused into the diamond to give it an electricalconductivity; as the second step, the bonding phase component melts andfills the space between the boron-doped diamond powder particles; andthen the diamond sintered compact having good electric conductivity isproduced by this simple and effective two step process. Therefore, thediamond sintered compact produced by this aspect 2 of the presentinvention has good electrical conductivity as well as good thermalresistance. Additionally, this sintered compact has the goodcharacteristics, namely, good hardness, good thermal conductivity andgood chemical stability, almost equal to those of natural diamonds.Thereby, this diamond sintered compact has an excellent machinabilitythat the electrical discharge machining process is easily possible touse for machining this diamond sintered compact. As this boron-dopeddiamond sintered compact will be expecting wide application in variousindustrial fields, the practical effects brought by the presentinvention are very significant.

1. A boron-doped diamond sintered compact comprising: a quantity in therange of from 90 to 99.9 weight % of a boron-doped diamond powder inwhich the range of from 1 to 10 wt-% of boron component is doped, and aquantity in the range of from 0.1 to 10 wt-% of an alkaline earth metalcarbonate powder acting as a bonding phase component in this sinteredcompact, comprising: one or more carbonates including alkaline earthmetal elements Mg (magnesium), Ca (calcium), Sr (strontium) or Ba(barium); and/or one of more composite carbonates including two or moreof the alkaline earth metal elements; wherein the boron-doped diamondpowder and the alkaline earth metal carbonate powder are sinteredtogether under HP/HT conditions at a pressure from 6.0 to 9.0 GPa and atemperature is from 1600 to 2500° C.; and wherein the bonding phasecomponent melts and fills space between the boron-doped diamond powderparticles.
 2. A production method for a boron-doped diamond sinteredcompact comprising the steps of: providing a quantity in the range offrom 90 to 99.9 wt-% of a boron-doped diamond powder in which the rangeof from 1 to 10 wt-% of boron component is doped, providing a quantityin the range of from 0.1 to 10 wt-% of an alkaline earth metal carbonatepowder acting as a bonding phase component in this sintered compactcomprising one or more carbonates including alkaline earth metalelements Mg (magnesium), Ca (calcium), Sr (strontium) or Ba (barium),and/or one or more composite carbonates including two or more of thealkaline earth metal elements; and sintering these boron-doped diamondpowder and the alkaline earth metal carbonate powder together in a HP/HTapparatus, under the conditions of a pressure in the range of from 6.0to 9.0 GPa and a temperature in the range of from 1600 to 2500° C.;wherein the bonding phase component melts and fills space between theboron-doped diamond powder particles.
 3. A production method for adiamond sintered compact having good electrical conductivity comprisingthe steps of: providing a mixture of a quantity of diamond powder in therange of from 80 to 99.4 wt-% and a quantity of boron powder in therange of from 0.5 to 15 wt-%, and providing a quantity in the range offrom 0.1 to 10 wt-% of an alkaline earth metal carbonate powder as abonding phase component in the sintered compact comprising one or morecarbonates including alkaline earth metal elements Mg (magnesium), Ca(calcium), Sr (strontium) or Ba (barium), and/or one or more compositecarbonates including two or more of the alkaline earth metal elements,;sintering in HP/HT apparatus the diamond, boron, and alkaline earthmetal carbonate powder such that the boron component in the mixture isdiffused into the diamond powder under the conditions of a pressure inthe range of from 5.0 to 8.0 GPa and a temperature in the range of from1300 to 1800° C., and melting the bonding phase component under theconditions of a pressure in the range of from 6.0 to 9.0 GPa and atemperature in the range of from 1600 to 2500° C., such that the bondingphase fills space between the boron-doped diamond powder particles.
 4. Aboron-doped diamond sintered compact according to claim 1, wherein thecompact has electric conductivity of 70 Ω·cm or lower.
 5. A boron-dopeddiamond sintered compact according to claim 1, wherein the compactproduces no graphite after being heated in a vacuum furnace at atemperature of 800° C. for 30 minutes and then at a temperature of 1200°C. for 30 minutes.
 6. A boron-doped diamond sintered compact accordingto claim 1, wherein the compact presents no shape deformation afterbeing soaked in heated fluoric acid at a temperature of 150° C. for 2hours.